This invention relates to a diode structure and method for its manufacture, and more specifically relates to a novel diode which has a reduced reverse recovery voltage characteristic and a soft turn-off characteristic.
Semiconductor diodes are well known, and are commonly made by diffusing a deep P/N junction into the surface of a monocrystalline silicon substrate. For example, a wafer of silicon having an epitaxially formed N type top layer may have a deep P diffusion formed in its top surface. Such diffusions are typically 4-20 xcexcm microns deep. Anode and cathode electrodes are then directly or indirectly connected to the P and N regions respectively. A suitable termination structure is also provided. Heavy metals such as gold and platinum may be diffused into the body of the silicon to xe2x80x9ckillxe2x80x9d or reduce the lifetime of minority carriers, thus increasing the turn-off speed of the device. Alternatively, E-beam radiation can be used for lifetime killing alone or with the heavy metals.
Such diodes have turn-off characteristics which can be detrimental in certain circuit applications. For example, when the diode becomes reverse biased for turnoff, the current will reverse through zero and then recover during a time to. As the current rises toward zero, it will have a recovery rate di/dt that will induce high voltage spikes in inductive circuits. U.S. Pat. No. 5,747,872 describes a soft turn-off diode, employing highly doped N and P layers separated by a lightly doped N layer and using heavy metal diffusion and irradiation.
It would be very desirable to have a diode with a xe2x80x9csoftxe2x80x9d turn off characteristic and reduced recovery voltage characteristics and of simple structure for use in inductive circuits such as power factor correction circuits. Such a diode would reduce the production of high voltage spikes during diode turn-off in such circuits.
In accordance with the invention, a diode is made with a xe2x80x9cweakxe2x80x9d anode, that is, an intentionally weak injecting anode. Such an anode will produce a diode with soft turn-off characteristics.
Weak anodes, sometimes termed xe2x80x9ctransparentxe2x80x9d anodes, are known for use with non-punch through insulated gate bipolar transistors (xe2x80x9cNPT IGBTsxe2x80x9d) where they are used as a collector region of reduced injection efficiency. Such anodes are used to replace the need for buffer layers and lifetime killing in NPT IGBTs. These device will have a DMOS type top structure in a wafer, and the weak anode in the bottom surface of an ultra thin float zone silicon wafer.
In accordance with the invention, the conventional deep and heavily doped P diffusion into N type silicon of a conventional diode is replaced by a much shallower weak anode. The weak anode can be formed by a very shallow (0.5 micron) boron implant having a dose of about 1E14 cmxe2x88x922 at 80 KeV. It should be noted that only about 5% to 10% of the implanted dopant is electrically active because a conventional activation procedure of annealing at above about 700xc2x0 C. is not used. Other doses can be used, in the range of 1E11 to 1E16, depending on the forward voltage drop and reverse recovery required.
An aluminum layer is then sputtered or evaporated atop the boron implant and is sintered at about 420xc2x0 C. for 30-60 minutes. This may be followed with E-beam radiation of the wafer to further reduce the minority carrier lifetime.
Alternatively, the weak anode can be formed by the deposition of P type amorphous silicon on the wafer surface, as disclosed in copending U.S. application Ser. No. 09/566219, filed May 5, 2000 (IR-1462). The silicon surface receiving the weak anode may also be heated in vacuum as disclosed in copending application Ser. No. 09/565928 (IR-1706) to eliminate the need for the anneal step to activate the weak anode junction. The spaced activated weak anode described in copending U.S. application Ser. No. 09/565922 (IR-1708) may also be used.
The resulting diode formed by any of these processes will have a soft turn-off characteristic and reduced recovery charge. These characteristics make it ideal for use in motor control circuits, and as a power factor correction diode.